Structure and Roles of the Various Layers in the Shells of Conch Conus Litteratus

Journal of Bionic Engineering 13 (2016) 124–131

Structure and Roles of the Various Layers in the Shells of Conch Conus litteratus

Xiaojuan Hong, Xiaoxiang Wang College of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

Abstract Mollusc shells are renowned for their mechanical strength and toughness. To better understand the mineralization process of the shell, structure of the body whorl and base of Conus litteratus (Conus shell) were in detail investigated by using scanning electron microscopy. Three-point bending tests were taken to demonstrate that each layer of crossed-lamellar structures is indispensable to enhance the whole strength of the shells. The results show that the conch shell is composed of hierarchical structure from nano scale to macro scale, and the basic constituent is long rod-shaped aragonite. Different positions of the shell have varied structures, and the base is more complicated than the body whorl. The mechanical properties of Conus are highly anisotropic and the arrangement of middle layer has a great influence on the bending strength. The outer and inner layers are very thin but play a protective role for the middle layer. Keywords: crossed-lamellar, mechanical properties, aragonite, mollusc shell Copyright © 2016, Jilin University. Published by Elsevier Limited and Science Press. All rights reserved. doi: 10.1016/S1672-6529(14)60166-9

The novel structure and mechanical property of the Nomenclature nacre have become a new research trend during the past TP Transverse sample, loading on the P plane few decades[9−12]. Nacre is composed of a brick-and- LP Longitudinal sample, loading on the P plane mortar like structure and is considered to be stronger TP-M Middle layer of TP than the other structures in the shells[13]. However, an- LP-M Middle layer of LP other typical structure—crossed-lamellar is less strong TP-OM Transverse sample that machined off the but more widely exists in nature[14,15]. Crossed-lamellar inner layer has the property of fast growth and low content protein TP-MI Transverse sample that machined off the (less than 1%), and its repair processes are quicker than outer layer nacre when the shell is damaged[16]. Crossed-lamellar also has excellent mechanical properties that even sur- 1 Introduction pass those of nacre in some respects. The researches of The shell material has attracted increasing attention conch shell have provided clear evidence that the [17] of many scientists due to its high-toughness and inter- crossed-lamellar structure is the hardest and its rup- [1,3] mediate-strength, which can surpass man-made ceram- ture work can reach 10 times as high as that of nacre . ics[1–6]. Shells are ceramic/polymer laminated compos- Conus shells generally are made of three crossed layers on the macro[18]. Current researches of Conus mainly ites consisting of aligned, anisometric, calcium (CaCO3) grains separated by extremely thin protein matrix[7,8]. focus on the partial structure and mechanical properties, Structure is essentially crucial to the mechanical prop- trying to explain why these mollusc shells have high erties when the components are consistent. The design fracture work. The main mechanisms of high toughness principles found in these natural materials inspires sci- have been summarized: extensive crack deflection, fiber [15,19,20] entists to synthesize composite materials owing opti- pull out and mineral bridges . mized properties. As is known to all, the great mass of the shell pro-

Corresponding author: Xiaoxiang Wang E-mail: [email protected]

Hong and Wang: Structure and Roles of the Various Layers in the Shells of Conch Conus litteratus 125 vides stability against upheaval by tidal forces or minimize varying test results caused by the influence of predatory attackers, as well as protection from crab at- age and environment, all longitudinal and transverse tacks that may involve forces as great as 800 N[7]. But samples on each contrast experiments were cut out of scars are found in the body whorl in a number of conus one shell. Hacksaw with an abrasive blade were used to shells (Fig. 1). To better understand the design of mol- obtain rectangles with approximate sizes firstly. Suc- lusc shells, the holistic structure of conus shells and cessively, these precut specimens were divided into mechanical properties that over different directions have three groups and machined by raw abrasive papers to get been conducted in this paper. It proposes the possibility the desired size for the following experiments. The that the source of the scars in the shells. length of 8 mm and different sizes of cross sectional areas were kept, considering the rectangular samples of 2 Materials and methods transverse were too hard to take. The span was 6 mm for Conus litteratus was studied herein and the ap- all the tests and the samples herein were tested by a head pearance is shown in Fig. 1. The sampling section is the speed of 0.1 mm·min−1. The results can only give com- body whorl of the conch shell, where symbols L, T and P parison within group because of the samples are not represent longitudinal section, transverse section and standard. stratification plane, respectively. Integrated samples, which have the full thickness of the shell with the sectional areas about 2 mm × 2.5 mm, 2.1 Scanning Electron Microscopy(SEM) were tested on different planes in the first group. The The intact longitudinal section of the body whorl loading was on P plane for the transverse and longitu- and cross section of the base were prepared. The samples dinal samples (TP, LP). Fig. 5a gives an overview of the were polished by abrasive papers and etched by 1% HCl different test configurations. In the second group, all for 15 s. Subsequently, the structures of Conus litteratus samples were machined off the inner layer and outer on different positions were observed by SEM (S4800) layer to remain the middle layer with cross sectional after spray silver treatment. Fracture morphologies of areas about 1 mm × 1 mm. The loading was applied on P some areas were also taken by SEM. plane for the transverse specimens (TP-M) and longitudinal specimens (LP-M). In the third group, the trans- 2.2 Three-point bending tests verse samples were tested on P plane (TP). A part of These tests were desired to declare the role of each them were removed the inner layer (TP-OM) or outer layer on promoting the whole strength of the shell. To layer (TP-MI).

3 Results and discussion 3.1 Structure of the body whorl 3.1.1 Macrostructure A mature conch Conus litteratus has a cone-like shell of proximate 60 mm – 70 mm high and of 30 mm – 40 mm diameter base, as shown in Fig. 1. The longitudinal section of the shell (Fig. 2a) indicates that the shell consists of several whorls growing from inside to outside. The most outside whorl is quite uniform in thickness and is also the thickest, and the middle position is about 2.0 mm – 3.0 mm. Due to this reason, we used the shell of this whorl as sample to measure the mechanical properties of the shell. The uniform area of the shell is constituted of three Fig. 1 The appearance of Conus litteratus, showing the cutting layers along the thickness direction: outer layer (O), directions and positions of the test pieces; Scar is shown in the right one. L: longitudinal section, T: transverse section, P: strati- middle layer (M) and inner layer (I) (Fig. 2d). The mid- fication plane. dle layer is the thickest, about 1.5 mm, while the inner Download English Version: https://daneshyari.com/en/article/826543

Download Persian Version:

https://daneshyari.com/article/826543

Daneshyari.com